From Ethan Siegel: “How would our Universe be different without dark energy?”

From Ethan Siegel


Image credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona, of the Hercules Galaxy Cluster, under a c.c.a.-s.a.-4.0 license.

U Arizona Mt Lemmon Sky Center, north of Tuscon, AZ, USA
U Arizona Mt Lemmon Sky Center, north of Tuscon, AZ, USA

In 1998, cosmologists got the surprise of a lifetime. Here’s how our Universe would’ve looked without cosmic acceleration.

“We’ve known for a long time that the universe is expanding. But about 15 years ago, my colleagues and I discovered that it is expanding faster and faster. That is, the universe is accelerating, and that was not expected, but it is now attributed to this mysterious stuff called dark energy which seems to make up about 70 percent of the universe.” -Adam Riess

In 1998, two independent groups of scientists both studying the most distant supernova explosions in the Universe reported the same unexpected phenomenon: these brilliant flashes of light, whose intrinsic brightnesses and redshifts were known to great precision, all had a problem, that they appeared to be much fainter than expected. And the higher of a redshift you went to, the greater this problem got. The interpretation? They were more distant — and hence appeared less bright — than the conventional version of the expanding Universe would have predicted. Rather than being filled only with matter and radiation throughout the fabric of space, the Universe also contained this small but important amount of energy inherent to space itself: dark energy.

Image credit: Ned Wright, based on the latest data from Betoule et al. (2014), via

As our measurements got better and better, and as we accumulated data from other sources as well, like the fluctuations in the Cosmic Microwave Background (CMB) and the clustering properties of large-scale structure, we found out that approximately 68% of the energy in the Universe today was this mysterious dark energy. Yes, there was dark matter, normal matter, neutrinos and radiation all present, and they were all vital to how the Universe expanded and evolved, particularly at early times. But as the Universe aged, dark energy became more and more important, and will eventually approach a full 100% of the energy present within our Universe.

Constraints on dark energy from three independent sources: supernovae, the CMB and BAO. Note that even without supernovae, we’d need dark energy. Image credit: Supernova Cosmology Project, Amanullah, et al., Ap.J. (2010).

But according to General Relativity, it didn’t have to be this way at all. We could have had a Universe with no dark energy at all: where zero-point energy of empty space was actually zero, instead of some tiny, non-zero value. If that were our Universe, how would it be different from the Universe we have today? Surprisingly, there are a few significant ways that really make an impact.

A Universe with dark energy: our Universe. Image credit: NASA / WMAP Science Team.

1.) The Universe would be a little bit different. Right now, in our 13.8 billion year old Universe, 32% of the energy density is in the form of matter, 68% is dark energy, the expansion rate is 67 km/s/Mpc and the limits of our observable reach is 46.1 billion light years. If wanted the Universe to have the same exact amount of matter in it, but with no dark energy, our Universe would have expanded faster early on, and would be expanding slower today. It would:

be 47.7 billion light years in size, rather than 46.1 billion,
have a current Hubble rate of 56 km/s/Mpc rather than 67 km/s/Mpc,
the CMB temperature would be just slightly lower, at 2.62 K instead of 2.73 K,
and have a whopping 71% less energy overall, due to the total lack of dark energy.

But the major differences would show up far in the future, especially when we considered our eventual fates.

The GOODS-N field, with galaxy GN-z11 highlighted: the presently most-distant galaxy ever discovered. Image credit: NASA, ESA, P. Oesch (Yale University), G. Brammer (STScI), P. van Dokkum (Yale University), and G. Illingworth (University of California, Santa Cruz).

2.) Every galaxy in the visible Universe would still be reachable.

Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey
Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

In our dark-energy dominated Universe, the rate a distant galaxy recedes from us increases as time goes on. Galaxies presently more than 15 billion light years away are receding faster than the speed of light [?], and so nothing leaving Earth today — not a relativistic spaceship, not a deep-space probe, not even light itself — could ever reach it. Already, 97% of the galaxies in our Universe are forever beyond our reach. But if we were to take that dark energy away, everything would be reachable eventually, even if it took tens or hundreds of billions of years. We’d get there in the end.

A portion of the Hubble eXtreme Deep Field in full UV-vis-IR light, the deepest image ever obtained. Image credit: NASA, ESA, H. Teplitz and M. Rafelski (IPAC/Caltech), A. Koekemoer (STScI), R. Windhorst (Arizona State University), and Z. Levay (STScI).

3.) New galaxies beyond our horizon would continually become accessible. Not only that, but even galaxies whose light has never reached us yet will someday have that light catch up to us in the future! While a dark energy Universe has the currently visible galaxies “red out,” or redshift away to the point where they’ll no longer be seen in the far future, a Universe without it would’ve seen additional galaxies become visible over time, with more and more becoming apparent (and reachable) as time goes on.

Without dark energy, we’d be somewhere in between a decelerating and a coasting Universe. Image credit: NASA & ESA, of possible models of the expanding Universe.

4.) The Hubble rate of expansion would eventually drop to zero. It would never actually reach zero, mind you, and it would never turn around and recollapse: there’s too little energy for that overall. But the Hubble rate would asymptotically approach zero as the Universe continued to expand, meaning that if an infinite amount of time were to pass, an infinite number of galaxies (though not all of them, by any means) would become accessible. With dark energy, our Universe’s Hubble rate will asymptote to a finite, significant value after an infinite amount of time: something like 46 km/s/Mpc. Without dark energy, we’d have dropped below that 46 km/s/Mpc rate after another 4.3 billion years.

Laniakea supercluster no image credit
Laniakea supercluster. From Nature The Laniakea supercluster of galaxies R. Brent Tully, Hélène Courtois, Yehuda Hoffman & Daniel Pomarède at Milky Way is the red dot.

5.) Superclusters would really exist. Our local supercluster, containing the local group, the Virgo Cluster (the largest supercluster member) and hundreds of other individual galaxies, groups and clusters, doesn’t really exist thanks to dark energy.

Local Group. Andrew Z. Colvin 3 March 2011
Local Group. Andrew Z. Colvin 3 March 2011

Virgo Supercluster
Virgo Supercluster. No image credit

It looks like a large structure, but it’s not bound together and will have all of its individual components strewn apart as time goes on. But without that additional repulsion that dark energy imparts, gravitation would win in the end. On long enough timescales, all galaxies, groups and clusters that make up the Laniakea supercluster will remain bound together, and will continue to experience mergers on cosmic scales.

6.) Which means eventually, Milkdromeda would fall into the Virgo Cluster. At 50–60 million light years distant, the Virgo Cluster contains around 1000 galaxies, and is the closest galaxy cluster to our local group. It’s currently receding from us, due to the expansion of the Universe, at over 1000 km/s, or about 100 times faster than any human-made spacecraft has ever traveled. With dark energy, Virgo will only accelerate away from us faster and faster. But if it weren’t there, the gravitational pull of Virgo would be irresistible, and even though it would take around a hundred billion years — many times the age of the Universe at present — eventually the galactic wreckage of our local group would merge with the Virgo cluster as well.

Image credit: E. Siegel, based on work by Wikimedia Commons users Andrew Colvin 429 and Frédéric MICHEL.

With dark energy, the subtle differences of a slightly more energetic and more rapidly expanding Universe today leads to a far future where our local group is lonely and isolated, distant galaxies disappear from view and there’s no such thing as a bound, cosmic supercluster. On the largest scales, the Universe is doomed to emptiness, and it’s extra energy intrinsic to space itself that’s to blame. Part of why it was so hard to accept is because the fate of a dark energy Universe is so different — and unpalatable — from a Universe without it. Yet science doesn’t care about your personal preferences or motivations: it cares about the Universe as it actually is. The best thing we can do is listen to the story it tells us about itself, and in a way, about ourselves, too.

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“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan